This invention relates to an improvement in a EGR (Exhaust Gas Recirculation) control system for controlling recirculating a part of the exhaust gases passing through the exhaust gas passageway of an internal combustion engine back to the combustion chambers of the same.
It is well known in the art that a part of the exhaust gases of an internal combustion engine is recirculated back to the combustion chambers of the engine in order to suppress the maximum temperature of the combustion taken place in the combustion chambers to reduce the emission level of nitrogen oxides (NOx) generated during the combustion in the combustion chambers. By virtue of this exhaust gas recirculation, the NOx emission level has thus effectively been lowered. However, the recirculated exhaust gas greatly affects the combustion in the combustion chambers and stability of the engine. Therefore its amount is desired to be strictly controlled in consideration of vehicle driveability and fuel economy.
In this regard, it seems desirable to maintain constant EGR rate (the rate of the amount of recirculated exhaust gases with respect to the amount of intake air inducted to the engine). To meet this demand, the following EGR control system has been proposed by the same applicant as the present invention: an EGR control valve is operatively disposed in an EGR passageway downstream of a flow restricting orifice for regulating exhaust gas flow therethrough. The EGR control valve is controlled by a control vacuum which is made in a pressure regulating valve assembly. The pressure regulating valve assembly is constructed and arranged to dilute intake vacuum in an intake passageway for operating EGR control valve in accordance with the relationship between venturi vacuum and the EGR passageway between the EGR control valve and the flow restricting orifice.
By this EGR control system, the flow amount of recirculated exhaust gases is prevented from being affected by the variation of exhaust gas pressure. Accordingly, the recirculated exhaust gas amount can be controlled only in accordance with the venturi vacuum which is highly reliable as a function of the flow amount of the intake air conducted into the engine. This results in precise control of recirculated exhaust gases in accordance with intake air.
Now, the above-mentioned pressure regulating valve assembly is provided with a plurality of diaphragm members one of which communicates with the EGR passageway through a connecting passage. The connecting passage is provided thereinside with a flow restricting orifice for the purpose of lightening the action of exhaust gas pulsation to prevent the diaphragm from being damaged due to the exhaust pressure pulsation. However, such a flow restricting orifice in the connecting passage is, in general, has a small diameter and therefore it is liable to be clogged with foreign substances such as carbon particles in exhaust gases. Thus it is impossible to achieve precise control of exhaust gas recirculation.